As described later, various characteristics are required of lubricating oils depending on the use thereof in the field of so-called industrial lubricating oils.
For example, in the field of refrigerating machine oils, CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon), which have been conventionally used as a refrigerant for refrigeration/air conditioning equipments, have become an object of regulations due to the problem of the recent ozone depletion, and HFC (hydrofluorocarbon) has come to be used as a refrigerant in place of these.
Meanwhile, the above-mentioned HFC refrigerants still involve problems such as high global warming potential. Therefore, as alternative refrigerants for these freon refrigerants, use of natural refrigerants such as carbon dioxide (CO2) refrigerant or hydrocarbon refrigerants has been studied.
As refrigerating machine oils for HFC refrigerants, oxygen containing synthetic oils such as PAG (polyalkylene glycol), POE (polyol ester) and PVE (polyvinyl ether) which are compatible to HFC refrigerants have been conventionally used, but these oxygen containing synthetic oils have both drawback and advantage in the characteristics as a refrigerating machine oil. On the other hand, alkylbenzenes such as branched-chain alkylbenzenes are incompatible with HFC refrigerants but they have characteristics that they are superior to the oxygen containing synthetic oils in abrasion resistance and friction characteristics in the presence of a refrigerant (for example, see the following Patent Documents 1 and 2).
In the meantime, various refrigerating machine oils have been suggested as refrigerating machine oils for natural refrigerants. For example, as refrigerating machine oils for carbon dioxide refrigerants, Patent Document 3 below discloses those using carbon hydride type base oils such as alkylbenzene and poly-α-olefin, Patent Document 4 below discloses those using ether type base oils such as polyalkylene glycol and polyvinyl ether, and Patent Documents 5 to 7 below disclose those using ester type base oils, respectively.
In addition, lubricating oils used for gas compressors such as rotary gas compressors (compressor oils) are required to have excellent heat/oxidation stability for reasons that they are circulated and used and that they inevitably contact with a high temperature compressed gas. Owing to this, compressor oils in which a highly refined mineral oil type base oil or a synthetic hydrocarbon oil represented by a hydrogenated product of a poly-α-olefin is combined with a phenolic antioxidant such as 2,6-di-tert-butyl-p-cresol or an amine antioxidant such as phenyl-α-naphthylamine are generally used conventionally.
However, in order to attain sufficient heat/oxidation stability in lubricating oils such as rotary gas compressor oils in which heat/oxidation stability at high temperatures is deemed important, a large amount of the antioxidant must be added and in this case, there is caused a problem that the antioxidant itself is easy to become sludge. The resulting sludge may adhere to the bearing of the rotation part of the rotary gas compressor and cause heating and damage of the bearing and further lead to clogging of mist collection mechanism for separating compressed gas and oil mist (demister), which may force shutdown of the facilities.
In order to cope with this, formulations of additives for attaining both heat/oxidation stability and sludge resistance of the lubricating oil have been studied, and use of specific antioxidants such as p-branched-chain-alkylphenyl-α-naphthylamine has been suggested (for example, see Patent Document 8).
In the meantime, there are sliding parts involving metal-metal contact or metal-rubber (resin) contact in pumps, control valves, oil pressure cylinders and the like which constitute a hydraulic circuit. It is required that abrasion resistance and friction characteristics should be good in the hydraulic oil which takes a role as a lubricant for such sliding parts.
In addition, when sludge is resulted by deterioration of the hydraulic oil and generation of abrasion powder, increase in sliding resistance at the above-mentioned sliding parts and further clogging of flow control valves in the hydraulic circuit are caused, and thus, heat/oxidation stability as well as abrasion resistance and friction characteristics are required of the hydraulic oils.
Therefore, in the conventional hydraulic oils, various attempts have been made to meet the above-mentioned requirements. For example, in order to secure heat/oxidation stability of the hydraulic oils, highly refined mineral oils such as hydrofined mineral oils and hydrocracked mineral oils have been used as lubricating oil base oils, and besides, synthetic hydrocarbon oils such as poly-α-olefins have been used and further improvement in heat/oxidation stability has been attempted by adding a phenolic or amine antioxidant to the lubricating oil base oils. In addition, from the viewpoint of improvement in abrasion resistance, zinc containing abrasion inhibitors such as zinc dithiophosphate (ZnDTP) and zinc-free abrasion inhibitors such as phosphoric acid esters and amine salts thereof, thiophosphates and β-dithiophosphorylated propionic acid compounds have been used as abrasion inhibitors. Besides, from the viewpoint of improvement in friction characteristics, reduction of friction coefficient of the sliding surface has been attempted by combining a friction reduction agent with a hydraulic oil (for example, see Patent Documents 9 to 12).
In the meantime, metalworking oils have been conventionally used to lubricate processing parts of processed metal products in the field of metalwork. Characteristics which enable reduction of processing force, improvement in productivity, improvement in surface appearance (for example, luster after the rolling) of the processed products by good lubrication (hereinbelow referred to as “workability”) are required of such metalworking oils.
In order to cope with this, conventional metalworking oils added with additives such as oiliness agents and extreme pressure agents have been generally used in order to improve workability (for example, see Patent Documents 13 and 14).
In the meantime, heat treating oils have been conventionally used in heat-treatment (quenching, etc.) to modify metal by heating and cooling.
Cooling process when a product to be treated such as steel materials is quenched with a heat treating oil is usually as follows.
First, when a product to be treated is put into a heat treating oil, the product to be treated is covered with vapor of the oil or cracked gas thereof. At this stage, cooling rate is slow since heat is hard to transfer due to the shielding effect of the vapor film.
Next, surface temperature of the product to be treated gradually decreases and when it reaches below a certain temperature, nucleate boiling of the oil occurs. This stage is called a boiling stage and shows extremely large chilling effect. The temperature at which the vapor film of the oil collapses and nucleate boiling starts is referred to as “characteristic temperature” in JIS K 2242 (heat treating oil), and it is considered that a heat treating oil having a higher characteristic temperature, namely a heat treating oil in which the time required to reach the characteristic temperature is shorter, is desirable to attain sufficient hardness.
As the surface temperature of the product to be treated approaches the boiling point of the oil, the boiling abates, and when the temperature passes the boiling point, boiling terminates and gentle cooling only by convection is performed. The cooling rate at this stage depends on viscosity of the heat treating oil and shows the higher cooling characteristics as the heat treating oil has the lower viscosity. Owing to this, use of a heat treating oil having a kinematic viscosity not more than 30 mm2/s at 40° C. is recommended in JIS K 2242 (heat treating oils), and particularly when a steel material having a low hardenability is to be treated, use of a heat treating oil having a still lower viscosity not more than 26 mm2/s at 40° C. is recommended.
As above, it has been conventionally considered that heat treating oils having a high characteristic temperature and a low viscosity are desirable in order to attain sufficient hardness. In the conventional heat treating oils, however, when the viscosity of a mineral oil used as a base oil of the heat treating oil is simply lowered, characteristic temperature also falls, and therefore, an attempt to raise the characteristic temperature by adding a cooling characteristics improver such as a copolymer of ethylene and an α-olefin to a mineral oil having a low viscosity (for example, see Patent Document 15).
In the field of machine tools, improvement in processing precision of parts is required, and in accompaniment with this requirement, improvement in the positioning precision in the sliding guide surface is required. Performance of the sliding guide surface oil is deeply related with positioning precision in the sliding guide surface, and stick-slip reduction as well as low friction (that is, small friction coefficient) is demanded. Furthermore, in the lubricating oil for machine tools, demands for long life and maintenance-free properties are also increasing.
Therefore, in the conventional lubricating oil for machine tools, various attempts have been made to meet the above-mentioned requirements. For example, phosphorus compounds such as phosphoric acid esters and amine compounds thereof, sulfur compounds such as sulfurized oils and fats, sulfurized esters and so on have been used as an additive to attain excellent friction characteristics (for example, see Patent Documents 16 to 20 below).
Besides, in order to secure heat/oxidation stability of the lubrication oils for machine tools, highly refined mineral oils such as hydrofined mineral oils and hydrocracked mineral oils as well as solvent refined mineral oils, and besides, synthetic hydrocarbon oils such as poly-α-olefins have been used as lubricating oil base oils (for example, see Patent Documents 21 to 24).
In addition, it is important that lubricating oils used for steam turbines, gas turbines, rotary gas compressors, hydraulic machinery can endure long-term use since they are used at high temperatures and circulated and used. Deposition of insoluble matters (sludge) occurring in lubricating oils are strongly adverse particularly to the facilities or the apparatus mentioned above. For example, when the deposited sludge ingredients stick to the bearing of the rotation part, they cause heating and will invite the damage of the bearing in the worst case. In addition, when sludge deposits, there may be caused problems in the operation including clogging of filters disposed in the circulation. Still further, shutdown of the apparatus is forced when sludge accumulates in the control valves to cause failure in the operation of the control system. Therefore, characteristics which make sludge hard to deposit (herein below referred to as “sludge suppressing properties”) as well as heat/oxidation stability are required of lubricating oils used in such fields.
Therefore, in the conventional lubricating oils used for steam turbines, gas turbines, rotary gas compressors, hydraulic machinery, improvement in heat/oxidation stability and sludge suppressing properties has been attempted by using highly refined mineral oils and synthetic hydrocarbon oils represented by hydrogenated product of poly-α-olefins as a base oil, and combining an antioxidant with such a base oil (for example, see the following Patent Document 25).    Patent Document 1: Japanese Patent Laid-Open No. 08-27478    Patent Document 2: Japanese Patent Laid-Open No. 08-27479    Patent Document 3: Japanese Patent Laid-Open No. 10-46168    Patent Document 4: Japanese Patent Laid-Open No. 10-46169    Patent Document 5: Japanese Patent Laid-Open No. 2000-104084    Patent Document 6: Japanese Patent Laid-Open No. 2000-169868    Patent Document 7: Japanese Patent Laid-Open No. 2000-169869    Patent Document 8: Japanese Patent Laid-Open No. 07-252489    Patent Document 9: Japanese Patent Laid-Open No. 04-68082    Patent Document 10: Japanese Patent Laid-Open No. 2000-303086    Patent Document 11: Japanese Patent Laid-Open No. 2002-129180    Patent Document 12: Japanese Patent Laid-Open No. 2002-129181    Patent Document 13: Japanese Patent Laid-Open No. 10-273685    Patent Document 14: Japanese Patent Laid-Open No. 2003-165994    Patent Document 15: Japanese Patent Laid-Open No. 05-279730    Patent Document 16: Japanese Patent Laid-Open No. S57-67693    Patent Document 17: Japanese Patent Laid-Open No. S51-74005    Patent Document 18: Japanese Patent Laid-Open No. 08-134488    Patent Document 19: Japanese Patent Laid-Open No. 08-209175    Patent Document 20: Japanese Patent Laid-Open No. 11-209775    Patent Document 21: Japanese Patent Laid-Open No. 04-68082    Patent Document 22: Japanese Patent Laid-Open No. 2000-303086    Patent Document 23: Japanese Patent Laid-Open No. 2002-129180    Patent Document 24: Japanese Patent Laid-Open No. 2002-129181    Patent Document 25: Japanese Patent Laid-Open No. 07-252489